Revealing mistake: When Tim Allen is looking at the reindeer, one reindeer's ear moves. At this point you can see the metal robot in the base of the ear.
Other mistake: Near the beginning, Scott and Charlie are flying in the sleigh down a road next to a truck. During the entire scene, the truck driver is constantly jerking the wheel to the right and left yet the truck remains in a perfectly straight course.
Revealing mistake: When Tim Allen slips on his snow-covered roof, you can see the "snow" wrinkle as he slides.
Continuity mistake: In the scene where Bernard the elf appears at the Miller's house near the end of the movie, there is a shot of him walking out of the kitchen with Neal's snack that the latter had been preparing earlier (a stuffed pita shell) and he's talking to Neal as though his mouth is full from having taken a bite. However, if you look at the pita, it is still whole - not a single bite taken.
Continuity mistake: Right after the part where Santa fell off the roof and Scott is yelling at Charlie to stop climbing up the ladder, you will notice their roof is slanted and has a significant amount of snow on it. However, from the top of the ladder Charlie exits off with complete ease (given the circumstances of the roof and ladder position, difficult for a skilled adult let alone 6-year-old Charlie) and walks onto the roof, and the roof portion is now obviously a flat surface.
Continuity mistake: During mother's patronising talk to Scott and Charlie (fairly at the beginning of the movie) her gloves at one stage both end up briefly in the other hand.
Factual error: Tim Allen drives a 1st generation Ford Taurus SHO (1989-91) in the movie, which was only available with a manual transmission. When he pulls up and parks at the Denny's, you can see his hand down on the console and the clicking sound of shifting an automatic from drive into park.
Continuity mistake: When Scott and Charlie are at Denny's, Scott picks up the eggnog placard from the table to ask the waitress about it. He then puts it back down in a different spot. However, in the shots of Charlie by himself (such as when he asks for chocolate milk), the placard is visible in the foreground, in the same place it was before Scott picked it up.
Continuity mistake: After being told that he cannot see Charlie till the next year Tim Allen steps into Charlie's house. Before he enters it's snowing, when you see him inside with Charlie in his arms you can see outside the dining room window that there is no snow, and when Charlie's mum goes outside it's snowing again.
Continuity mistake: When Scott Calvin hears something on the roof and he goes outside, he sees Santa on the roof. He calls to Santa, "hey you", and Santa turns around and slips. As he's sliding down the roof, and falls you can see where Santa slid on the roof, because there is a line of no snow, in the next scene, like five seconds later, there is snow where Santa slid.
Continuity mistake: When Scott is talking to Charlie in the Zoo,we can see a polar bear swimming around in the background. One minute it is just behind Charlie and the next it is behind Scott - there's no time between shots for it to have swam across.